Magnetic Diagnostics of Magnetic Island in LHD

Abstract

Characteristics of magnetic islands are investigated by magnetic diagnostics in the L arge Helical Device (LHD). The structure of the magnetic island with m/n = 1/ 1( where,m and n are poloidal and toroidal mode number, respectively) can be estimated from the perturbed magnetic field appearing when a magnetic island changes. To measure the toroidal profile of the perturbed magnetic field δb1 originating from the plasma, a toroidal array of magnetic flux loops is set up in the LHD. The toroidal profile of δb1 is then spatially Fourier decomposed to determine the amplitude of the n = 1c omponent, δb n=1 1 and its phase, φn=1 whic hc orrespond the change of the island width and the toroidal position of the X-point of the island, respectively. Therefore, the information about the magnetic island structure can be obtained from δb n=1 1 and φn=1 .I ncase the island width becomes larger than the seed island, measurements show that δb n=1 1 is non-zero and φn=1 is temporally constant. A non-zero δb n=1 1 can also be observed when the island width becomes smaller than the seed island. In this case, the angle φn=1 shifts by about π[rad] compared with the increasing case and the δb n=1 1 is limited to a certain value whic hc orresponding to the magnetic field suppressing the seed island.